The present invention relates to an image forming device that forms an image using an ink jet recording method.
Conventionally, this type of image forming device uses an ink jet recording method in which pulse signals are applied to a plurality of heaters, provided in an ink-filled nozzle, to heat them until ink boils to cause the bubble pressure to eject the ink. In an image forming device using this method, a plurality of such nozzles are arranged to constitute a head and a plurality of such heads (for example, each ejecting a color ink such as cyan, magenta, yellow, or black ink) are combined to form a full-color image.
In such an image forming device, the control circuit, which drives each head, is configured as shown in FIG. 18. This figure shows the configuration of only one head. In this figure, numerals 1801 and 1802 indicate shift registers, numerals 1803 and 1804 indicate latch circuits, numeral 1805 indicates a decoder circuit, numeral 1806 indicates an AND circuit, numeral 1807 indicate transistors, and numeral 1808 indicates heaters. Image data VDO1 and VDO2, sent from an external unit in the form of serial binary data in synchronization with a transfer clock CLK, are sequentially converted from serial to parallel by the shift registers 1801 and 1802. Eight units of image data VDO1 and VDO2 are transferred and then latched in the latches 1803 and 1804 by the LAT signal. A head, composed of a plurality of nozzles, is divided into n blocks (in this example, a 256-nozzle head is divided into 16 blocks). One enable signal, BE0-BE15, and a heater driving signal HE are given to a block to turn on the transistors of the nozzles with which image data is stored in the enable state. These signals heat the heaters of the nozzles to eject the ink. In the image forming device, the block enable signal BE is converted from 4-bit code data to 16-bit data by the decoder 1805. When the block enable signal BE, image data VDO1 and VDO2 each composed of eight units of data, and the heater driving pulse signal HE are all turned on, the ink is ejected.
One column of data is printed as shown in FIG. 19 by this control. Repeating this operation for the number of columns in the main scanning direction prints one band of data. The paper sheet is fed one band to print the second band of data. This operation is repeated a lot of times to form an entire image composed of a lot of bands.
To print data in precise positions even when the carriage speed changes, a linear scale 109 with slits each for one or several dots is usually provided in parallel with the carriage movement path as shown in FIG. 20. A sensor 2003, provided near a head 101, reads this scale and outputs a signal to synchronize ink ejection so that the ink is ejected in correct positions.
However, unevenness in the shape or in the direction of ejection apertures causes unevenness in horizontal and vertical ink ejection positions on paper. At the same time, the heater size and contaminants near the nozzles cause unevenness in amount of the ink ejected per nozzle. When an image is printed at the same density with a print head composed of those recording elements, the image is printed not evenly but there is an unevenness in density. For example, as shown in the example in FIG. 21, an attempt to record a pattern at the 50% of image density results in uneven density printing depending upon the positions of nozzles on the print head.
A technology, called head shading, is proposed as means for correcting this uneven density printing. This technology is such that a check is made for density unevenness in image data recorded by all recording elements of the recording head at the same density and, based on the density unevenness, the density of image data output from each nozzle is adjusted.
For example, in position A in the head width direction, shown in FIG. 21, where the density is higher than the intended density of image data, the density level of the image to be output with that nozzle is decreased in advance. Conversely, in position B in the figure where the actual recording density is lower than the density intended by the image signal, the density level of the image to be output with that nozzle is increased in advance. This adjustment significantly reduces an unevenness in recording density caused by the recording head.
There are two means for checking and correcting the recording density: auto head shading and manual head shading. In the auto head shading, a recorded image pattern is read by a scanner or some other unit provided in the recording unit for automatic detection and correction of density unevenness. In the manual head shading, a user visually checks a recorded image pattern to determine the density unevenness and correction values.
When the auto head shading is performed on the image forming device described above, either a scanner separate from the device or a sensor built in the device is used to read the print result of a predetermined pattern printed by the device to check for an uneven density. At this time, in the method in which a scanner is used, the paper sheet on which the predetermined pattern has been printed must be taken out of the output section of the device and then placed on the glass window of the scanner. The printed paper sheet placed on the glass window is pressed flat by the cover, and the printed pattern is read by a high-resolution CCD line sensor. However, this method requires user""s intervention from the time the predetermined pattern is printed to the time the printed paper sheet is set on the glass window of the scanner, making the operation complex. To implement the device with a printer which has no scanner built in, there are a lot of problems: for example, the user must purchase a scanner separately and install a software product that reads image data from the scanner to analyze an unevenness density in the image data. Therefore, to implement the device with only a printer and to implement the auto head shading function, a sensor is preferably built in the printer to allow the sensor to read the printer pattern.
One of sensors that may be used in a printer is a CCD. However, a CCD has the problems given below.
The CCD and the light-emitting halogen lamp are expensive.
The CCD driving circuit and the output signal processing circuit are complex.
The use of a halogen lamp requires additional heat-insulating parts.
The problems described above make the unit larger.
Therefore, the CCD, if used in the device only to implement the auto head shading function, involves a lot of problems such as a larger unit size, increased cost, complex design, and so on. To avoid these problems, a low-cost reflective-type sensor is usually provided near the carriage to detect a printed head shading pattern with that reflective-type sensor.
However, when a reflective-type sensor is used to detect a pattern on the printed surface, a piece of paper is raised on the platen if the paper is too tough. Conversely, if a piece of paper is too soft, the paper becomes non-flat because cockles are sometimes generated thereon after the pattern is printed. In this state, reading the pattern with the reflective-type sensor causes changes in read-out signal level from the reference level (GND) depending on places on the paper surface even if the density level is constant. Therefore, even if correction data for auto head shading is calculated using this output, it is difficult to check the density unevenness correctly. The present invention seeks to solve the problems associated with the prior art described above. It is an object of the present invention to provide an image forming device capable of detecting the pattern density level correctly even if a print paper is raised or cockles are generated thereon.
To achieve the above objects, an image forming device according to the present invention is an image forming device using an ink jet recording method, the device forming a color image using a plurality of heads each of which has a plurality of ink ejection nozzles thereon, the device comprising means for printing a print pattern at a predetermined density using the plurality of ink ejection nozzles, one head at a time, on a print paper; a reflective-type optical sensor which reads the print pattern in each color while scanning the print pattern in a nozzle column direction; and density calculating means for calculating a density of the print pattern in each color based on an output of a reflective-type optical sensor, wherein the reflective-type optical sensor comprises a light emitting element which emits a light including all lights in red/blue/green regions on an optical wavelength and a plurality of light receiving elements each of which detects a light in one of the red/blue/green regions on optical wavelength, and wherein, based on an output of the light receiving element for a complementary color light of each pattern color and an output of a light receiving element for a non-complementary color light of the pattern color, the destiny calculating means calculates a difference between the two outputs to detect a density level on a nozzle position basis for the print pattern printed by each head.
In this way, the difference between the two outputs is calculated to detect the density level based on the output of the light receiving element for the complementary color light of each pattern color and the output of a light receiving element for a non-complementary color light of the pattern color. By doing so, even if a paper rise or a cockle is caused when the print pattern is printed on a print paper, their effects on the output of the light receiving elements are canceled. Therefore, this method enables the pattern density level to be calculated correctly.
For example, the print pattern is a band pattern recorded by all ink ejection nozzles of each head and having a width corresponding at least to a head width, the plurality of light receiving elements are arranged in a main scanning direction, the reflective-type optical sensor has a rectangular light transmission slit with longer sides aligned in the main scanning direction so that an image of the print pattern in a predetermined position is formed on receiving surfaces of the plurality of light receiving elements, and the reflective-type optical sensor scans relatively in a sub-scanning direction with respect to the pattern.
The density calculating means preferably calculates the density level on a nozzle basis for each head. Using the result, the print density of each head may be adjusted on a nozzle basis according to the density level calculated on a nozzle position basis for each head.